by
tlahren » Thu Apr 05, 2018 6:23 pm
James_Ball wrote:
We change the liner ever day, the gold seal about once a week and replace the 1m section of guard column about once a week, if it lasts that long. When replacing the guard we remove about 3-4cm of the analytical column to remove anything caught in the stationary phase that won't bake off. Ever few weeks we also swab out or even scrub out with a brush, the inlet. Every few months we have to replace the split vent lines too, it is amazing what contamination in that will do to cause loss of analytes.
I agree with James. I think in your case it may possibly be related to the split-vent lines. Especially if you are using a "pulsed splitless" injection method.
I saw once that I had lost almost all response on ONE internal standard compound for Method Blanks and LCS only. As soon as I injected a sample with "matrix" in it, the signal for that chemical would come back. I used a new liner about every 10 samples as we were running diluted soil extracts. I removed the split line from the inlet (Agilent 6890) and noticed what looked like black tar gooping out of the line and into the injection port. I cleaned this well with MeCl2 and Hexane and replaced it. The split-vent trap was clean as it was down the line farther and most of the "tar" condensed out in the first few inches of cold copper split-vent line. I was getting a selective adsorption of specific chemicals during the pulse injection. It was cured after cleaning the split-vent line.
I observed that standard splitless and Split are far less affected by this. If you can imagine, as the inlet pressure is regulated via back-pressure from the mass flow controller at the back of the instrument, a bit of your sample is actually pushed into the split-vent and pulled back into the injection port as the sample is expanding and vaporizing in the inlet. This is not the same as "back-flashing" when you inject more than the liner volume and your sample vapor goes up into the cold carrier gas line. In my case, this was not the problem.
I suggest cleaning/replacing the split-vent as James does. You may not need to do it as often but it's definitely considered a maintenance item. I would have a spare copper line (Agilent systems again) ready to go so you can swap them in a few minutes. I would leave the top covers off of my Agilent GC as they are a pain to take off and get back on.